1. Field of the Invention
The present invention relates to optical transmission communication and particularly to an optical transmission system connected with one or more optical repeating apparatuses, an optical repeating apparatus, and a quality supervising method for the same.
2. Description of Related Art
When deterioration in quality occurs in the service signal in the optical transmission system, a communication business company and a maintenance company of the communication facilities are required to search for a fault having generated such deterioration in transmission quality and to conduct the repair. However, if a level variation due to deterioration in transmission quality of service signal occurs momentarily, it is very difficult to identify the cause and defective area in the conventional optical transmission system. As an example of occurrence of a momentary level variation, stress is given in some cases to an optical fiber during the work in an optical line terminal.
FIG. 1 illustrates a structure of the conventional optical amplifying and repeating transmission system. The conventional optical amplifying and repeating transmission system comprises optical transmitters 901-1 to 901-n, optical amplifiers 902 to 905, optical receivers 906-1 to 906-n, a wavelength multiplexer 908, a wavelength demultiplexer 909, and optical fiber transmission lines 910 to 912. Numerals 921, 922 designate optical line terminals.
Next, with reference to FIG. 1, operations of the conventional optical amplifying and repeating transmission system will be described.
The optical transmitters 901-1 to 901-n in FIG. 1 output optical signals preset to the wavelengths λ1, λ2, . . . , λn in order to wavelength-multiplex. The outputted optical signals λ1, λ2, . . . , λn are wavelength-multiplexed with a wavelength multiplexer 908. The wavelength-multiplexed optical signals λ1 to λn are amplified at a time with the optical amplifier 902 and are then outputted to the optical fiber transmission line 910. The wavelength-multiplexed optical signals λ1 to λn which have been outputted are sequentially amplified with the optical amplifiers 903 to 905 in the course of transmission through the optical fiber transmission lines 910 to 912.
Accordingly, loss generated in the preceding optical fiber transmission line is compensated. The wavelength-multiplexed optical signals λ1 to λn which have been compensated in loss are then wavelength-demultiplexed with the wavelength demultiplexer 909 into the optical signals of single wavelengths λ1, λ2, . . . , λn. The demultiplexed optical signals λ1, λ2, . . . , λn are respectively received with the optical receivers 906-1 to 906-n.
Therefore, this conventional optical amplifying and repeating transmission system realizes the optical amplifying and repeating transmission through wavelength-multiplexing of a plurality of optical signals via a single optical fiber transmission line.
FIG. 2 is a time chart when a fault is generated in the conventional optical amplifying and repeating transmission system. If optical signal is lost due to disturbance for the optical fiber transmission lines 910 to 912 (for example, loss increases because an external pressure is applied to the optical fiber) and a fault generated in the optical amplifiers 902 to 905, then a power waveform of optical signal 1001 shows a variation in level 1003 for the time axis. The time in which such variation in level 1003 is generated continues to the order of 1 msec, in the shortest case, from the order of 100 msec.
As a method of supervising a bit error generated by such variation in optical signal at the optical line terminal in the receiving side, the method disclosed in “ITU-T Recommendation G. 783” has been known.
In order to detect such a fault, a means for detecting the level of optical signal being transmitted is provided within each optical amplifier in the conventional optical amplifying and repeating transmission system. Levels of the optical signals are supervised as described above. However, limitation is set to the capacity of memory for storing such supervisory information and the time for storing the same in the conventional optical amplifying and repeating transmission system.
Accordingly, as illustrated in FIG. 2, an interval (detection interval) Ts of the supervisory point 1002 has generally been set to about several tens of minutes. Therefore, if instantaneous level variation is generated in the order of about 100 msec from the order of 1 msec like the level variation 1003 at the time other than the supervisory point 1002, such level variation has not been detected. Therefore, if deterioration of signal quality is generated at the receiving terminal, it has been difficult to identify the cause and defective point.
In other words, an optical signal level is detected by an optical signal level detecting means provided in each optical amplifier 902 to 905. The detected information (optical signal level) is accumulated within the memory provided in each optical amplifier. Momentary optical level variation can be detected by setting the interval (detection interval) of the supervisory point 1002 in the detecting means as short as possible. However, an amount of accumulation (capacity) of memory is limited and period of accumulation is also limited.
Accordingly, if the detection interval is set short, then a detection amount exceeds the capacity of memory. Therefore, the detection interval can generally be set only to several tens of minutes, and thereby it has been difficult to detect momentary variation in the signal level. FIG. 2 also shows an example of no-detection that level variation 1003 is generated within the detection interval Ts (about several tens of minutes).
Even if such momentary level variation is generated, this level variation causes generation of a bit error at the optical receiving terminal. Therefore, prior to the invention, there has been no method of detecting momentary level variation in each optical amplifier provided in the optical fiber transmission line in the preceding stage of the optical receiving terminal.